More 3 phase 3 ungrounded systems

[meternerd]

One other comment...

One other comment...

I noticed in the OP that there is still a 5 jaw meter. To meter a grounded wye ststem, you have to use a form 16 meter, which is a 7 jaw meter. If the utility did really hook up the bank ungrounded wye, then there should be no ground connection to any of the three neutral bushings on the bank. A tank ground that is not bonded to the bushing is OK in most states. It is permissable to connect an ungrounded service [250.24 (E) and 250.20.(B) (1)] if , when grounded, the voltage to ground on any ungrounded phase would be more than 150V, but I doubt many utilities would. If there is a ground connection on the neutral, then all bets are off and they should have connected the bare neutral conductor at the transformer bank and made the customer install a fourth wire in the service riser. It should then be bonded in the service disconnect panel as stated in 250.24 Then they should send all of the linemen involved back to school. I'm betting they wired it 3 wire ungrounded wye. If that's the case, ground detectors are required per 250.21 (B)

 

[puckman]

Interesting post our head electrician said that a service at one of the buildings of the company has a 3 wire 480v corner grounded service . At that time we were going to shut down for repairs and go on generater power. A gen was brought in and all they did was connect 3 wires to the main and not connect the ground wire to the service. I know a call was placed to the gen people and they were told about the corner grounded service.
The gen was used for about an hour total and later on when repairs were completed we found [2]120v meters damaged.
Just a little comment.

 

[kwired]

Interesting post our head electrician said that a service at one of the buildings of the company has a 3 wire 480v corner grounded service . At that time we were going to shut down for repairs and go on generater power. A gen was brought in and all they did was connect 3 wires to the main and not connect the ground wire to the service. I know a call was placed to the gen people and they were told about the corner grounded service.
The gen was used for about an hour total and later on when repairs were completed we found [2]120v meters damaged.
Just a little comment.

It way likely a 277/480 wye generator. If so all they had to do was float the neutral and bring the three phases to your equipment. If connected ahead of the SBJ then the bond of the grounded phase is the SBJ (should have really run an EGC back to the generator to bond the frame but it still would have worked)

Any damage to 120 volt equipment was from some other cause and not because of issues of corner grounding. If the 120 volt transformer supplying that equipment has 480 on the primary you get 120 on the secondary. This equipment would be more subject to damage from voltage and frequency fluctuations associated with using a less stable source being the generator as compared to how stable voltage and frequency is coming from the utility.

 

[meternerd]

It way likely a 277/480 wye generator. If so all they had to do was float the neutral and bring the three phases to your equipment. If connected ahead of the SBJ then the bond of the grounded phase is the SBJ (should have really run an EGC back to the generator to bond the frame but it still would have worked)

Any damage to 120 volt equipment was from some other cause and not because of issues of corner grounding. If the 120 volt transformer supplying that equipment has 480 on the primary you get 120 on the secondary. This equipment would be more subject to damage from voltage and frequency fluctuations associated with using a less stable source being the generator as compared to how stable voltage and frequency is coming from the utility.

Hmmm...if the generator was wired 3 Ph 4 Wire with the neutral bonded to the ground of the generator, (which most are), then running a ground from the generator frame to the service would result in a 277 Volt to ground fault because one of the phases of the load is grounded. The only way it would work is if you made sure that the grounded phase was not grounded when being fed from the generator. If it didn't trip a breaker, if could have sent some pretty strange voltages out on the 480/120 transformer.

 

[kwired]

It way likely a 277/480 wye generator. If so all they had to do was float the neutral and bring the three phases to your equipment. If connected ahead of the SBJ then the bond of the grounded phase is the SBJ (should have really run an EGC back to the generator to bond the frame but it still would have worked)

Any damage to 120 volt equipment was from some other cause and not because of issues of corner grounding. If the 120 volt transformer supplying that equipment has 480 on the primary you get 120 on the secondary. This equipment would be more subject to damage from voltage and frequency fluctuations associated with using a less stable source being the generator as compared to how stable voltage and frequency is coming from the utility.

Hmmm...if the generator was wired 3 Ph 4 Wire with the neutral bonded to the ground of the generator, (which most are), then running a ground from the generator frame to the service would result in a 277 Volt to ground fault because one of the phases of the load is grounded. The only way it would work is if you made sure that the grounded phase was not grounded when being fed from the generator. If it didn't trip a breaker, if could have sent some pretty strange voltages out on the 480/120 transformer.

You do have to make sure the neutral is not grounded at the generator otherwise you will have a 277 volt short from neutral to whichever phase you grounded.

Why you going to have strange voltage on out put of 480/120 transfromer. If you input 480 you get 120 out. It will not care if one of the input leads is grounded or not. Output side is separately derived system and only cares that input side is supplied with correct voltage. Nothing else changes as far as the separately derived system goes.

 

[meternerd]

You do have to make sure the neutral is not grounded at the generator otherwise you will have a 277 volt short from neutral to whichever phase you grounded.

Why you going to have strange voltage on out put of 480/120 transfromer. If you input 480 you get 120 out. It will not care if one of the input leads is grounded or not. Output side is separately derived system and only cares that input side is supplied with correct voltage. Nothing else changes as far as the separately derived system goes.

Why assume that the 480 is not affected? Let's assume the generator was a grounded Wye with the neutral bonded to the generator frame, but the neutral left unconnected at the load panel.. Now, hook that into a corner grounded 480 delta system. If there is ANY ground path from the generator to the load, then current will flow due to the 277V difference between the ungrounded phases of the load to the grounded Wye center point of the generator. Then assume that the return current on the phase that is grounded sags in voltage enough to cause one phase of the 120/208 (assuming it a delta/wye transformer) secondary to sag as well. This sag would be load dependent. That could cause a low voltage condtion great enough to cause problems. If there was an equipment ground conductor run from the generator to the service panel, the problem would be much greater. There may or may not be enough current to trip the breaker, but there would most likely be enough to load up one phase, depending on the size of the generator. Lots of unknowns here, but damage on two 120 V devices sounds more than coincidental to me. Just thinking of explanations.

 

[kwired]

Why assume that the 480 is not affected? Let's assume the generator was a grounded Wye with the neutral bonded to the generator frame, but the neutral left unconnected at the load panel.. Now, hook that into a corner grounded 480 delta system. If there is ANY ground path from the generator to the load, then current will flow due to the 277V difference between the ungrounded phases of the load to the grounded Wye center point of the generator. Then assume that the return current on the phase that is grounded sags in voltage enough to cause one phase of the 120/208 (assuming it a delta/wye transformer) secondary to sag as well. This sag would be load dependent. That could cause a low voltage condtion great enough to cause problems. If there was an equipment ground conductor run from the generator to the service panel, the problem would be much greater. There may or may not be enough current to trip the breaker, but there would most likely be enough to load up one phase, depending on the size of the generator. Lots of unknowns here, but damage on two 120 V devices sounds more than coincidental to me. Just thinking of explanations.

How much current has to flow to get that kind of voltage sag? Hopefully enough to operate overcurrent protection on the generator. If not you will probably have a generator that needs rewinding on top of whatever else is damaged.

High enough fault current may stall the generator engine or other prime mover if overcurrent protection doesn't open soon enough.

I have seen tractor PTO driven generators break universal joints on the drive shaft because of a dead short. In these cases the tractor had well above the required HP needed for driving the generator. A smaller tractor may have just stalled from the load imposed by the short.